Automatic test equipment for checking printed circuit boards has long involved use of a "bed of nails" test fixture to which the circuit board is mounted during testing. This test fixture includes a large number of nail-like, spring loaded test probes arranged to make electrical contact under spring pressure with designated test points on the circuit board under test. Any particular circuit laid out on a printed circuit board is likely to be different from other circuits, and consequently, the bed of nails arrangement for contacting test points in a particular circuit board must be customized for that circuit board. When the circuit to be tested is designed, a pattern of test points to be used in checking it is selected, and a corresponding array of test probes is configured in the test fixture. This typically involves drilling a pattern of holes in a probe plate to match the customized array of test probes and then mounting the test probes in the drilled holes on the probe plate. The circuit board is then mounted in the fixture, superimposed on the array of test probes. During testing, the spring loaded test probes are brought into spring pressure contact with the test points on the circuit board under test. Electrical test signals are then transferred from the board to the test probes and then to the exterior of the fixture for communication with a high speed electronic test analyzer which detects continuity or lack of continuity between various test points in the circuits on the board.
Various approaches have been used in the past for bringing the test probes and the circuit board under test into pressure contact for testing. One class of these fixtures is a wired test fixture in which the test probes are individually wired to separate interface contacts for use in transmitting test signals from the probes to the external electronically controlled test analyzer. These wired test fixtures are often referred to as "vacuum test fixtures" since a vacuum is applied to the interior of the test fixture housing during testing to compress the circuit board into contact with the test probes. A lid on the housing is closed over the test fixture to form a vacuum seal between the lid and the probe plate. A vacuum applied to the region between the lid and probe plate then moves the lid toward the probe plate, which compresses the circuit board between the lid and the probe plate. This applies spring pressure from the probes to the board. Customized wired test fixtures of similar construction also can be made by using mechanical means, other than vacuum, to apply the spring force necessary for compressing the board into contact with the probes during testing.
If the circuits on only one side of the printed circuit board are tested, the test probes in a wired test fixture are typically mounted in the probe plate below the board under test. It is common to connect the bottoms of the test probes to corresponding pins on separate interface blocks located at the periphery of the fixture housing below the circuit board. These connections are made by separate wires which are wire wrapped, crimped, soldered, or otherwise electrically connected to the bottoms of the probes and to corresponding interface pins. During testing, the system interface pins are forced into pressure contact with corresponding contacts in a test head receiver which is part of the base system. This receiver communicates the electrical test signals received from the probes and the interface pins to the external computer controlled electronic test analyzer.
When a two-sided board is tested by such prior art wired test fixtures, fixturing is more complex. The test signals pass from connections at the bottom of the fixture interface pins to the external tester in a manner similar to testing a single-sided board; the complexity arises in transferring signals from the upper side of the board to connections at the bottom interface pins. More specifically, the same arrangement of test probes mounted in the probe plate can be used for contacting the bottom of the circuit board, and the same wired connections are used for transferring the test signals to the interface pins, the receiver, and then to the electronic test analyzer. For testing the circuits on the upper face of the same board, an array of test probes is mounted on an upper probe plate in a pattern to match the pattern of test points in the circuits on the topside of the board. When the upper probe plate is closed, the upper test probes are aligned with the circuits on the upper surface of the board. Transfer pins are placed in areas on the upper probe plate outside the region covered by the board under test. These transfer pins are wired to corresponding upper test probes. Additional transfer pins are located in areas of the lower probe plate outside the region covered by the circuit board. The transfer pins on the upper probe plate make contact with corresponding transfer pins on the lower probe plate during testing. These lower transfer pins are wired to further corresponding interface pins on interface blocks in the lower portion of the fixture. When a vacuum is applied, test signals are communicated from the upper test probes to the upper transfer pins and through to the lower transfer pins and then on to the system interface pins and the contacts in the test head receiver and then to the electronic test analyzer.
The wire-wrapping or other connection of test probes, interface pins and transfer pins is time intensive. Substantial costs can be saved by reducing the number of wired connections used in making a customized wired test fixture. The present invention provides a test fixture having improved dual side access for testing circuits on both sides of the circuit board. The improvements reduce the number of wires and wired connections and also reduce the amount of spring probes used in the fixture when compared with the conventional prior art dual side fixture described above. These advantages are accomplished without added complexity to the receiver and electronic test system.
As mentioned previously, the customized wired test fixtures are one class of fixtures for transmitting test signals from the fixture to the external circuit tester; and more particularly, vacuum fixtures are one category of wired test fixtures. A further class of test fixtures is the so-called grid-type fixture in which the test points on both sides of a board are contacted by flexible pins which can move to contact the random pattern of test points on the board and transfer test signals to sets of interface pins arranged in a grid pattern in the receiver. In these grid-type testers, fixturing is generally less complex and simpler than in the customized wired test fixtures; but with a grid system, the grid interfaces and test electronics are substantially more complex and costly. It is the customized wired test fixtures to which the present invention is generally directed, and the resulting cost savings in fixturing can make this type of test system economically more attractive than the grid type system.